1. Field of the Invention
The present invention relates to a process for the preparation of carbazole by dehydrogenation or aminating dehydrogenation of the compounds of the formulae (I) to (VIII) mentioned below in the presence of iridium catalysts or iridium-containing catalysts on supports.
Carbazole is a starting substance for the preparation of dyestuffs, polymers and insecticides (Ullmann's Encyclopedia, 5th edition, volume A5, pages 59-60).
2. Description of the Related Art
It is known that carbazole can be prepared by dehydrogenation or aminating dehydrogenation of compounds of the formulae (I) to (VIII) given below, in which parts of the carbazole skeleton are already preformed, with the aid of platinum catalysts. According to U.S. Pat. No. 2,921,942, platinum on Al.sub.2 O.sub.3 or on SiO.sub.2 as a support is employed in this process. Such catalysts lead to good yields of carbazole at acceptably high conversions, but after a short period of use of only a few tens of hours, even at such low loads as 0.1 g of diphenylamine per ml of catalyst and hour (g/ml.h) or even lower loads, they are already deactivated. As a result, the productivity of the catalyst, that is to say the amount of carbazole formed per hour and per ml of catalyst, even at high conversions of the diphenylamine employed, is so low that large amounts of catalyst and voluminous reactors are required for industrial purposes. Longer service lives between regenerations of the catalyst, that is to say those of several tens of hours, are obtained if magnesium oxide is chosen as the support for the platinum catalyst (U.S. Pat. No. 3,041,349). Nevertheless, the life of this catalyst is evidently only a little longer than about 150 to 200 hours in total (Table II in U.S. '349), since after a somewhat longer service life than in the above U.S. '942, that is to say a service life of about 60 or 62 hours (calculated according to the data of the diphenylamine (DPA) added in U.S. '349) between in each case two regenerations, the time before the next regeneration drops to about 17 hours, which without doubt indicates that this catalyst will soon become completely unusable. Here also, the load is only about 0.1 g/ml.h. Catalysts based on chromium oxide according to U.S. Pat. No. 3,085,095 likewise rapidly become unusable, although they tolerate somewhat higher loads.
The object of the present invention was accordingly the development of catalysts which result in good conversions and yields of carbazole under high loads and have high service lives between two regenerations, and overall a long life. It has been possible to achieve this object if iridium catalysts or iridium-containing catalysts in which other metals of the platinum metal group are employed, in addition to the iridium, are employed.